1. Field of the Invention
The present invention relates to a current-voltage converter that converts presence/absence of an input signal into a voltage signal when current wide in amplitude range is inputted. More particularly, it relates to a current-voltage converter used for the case after an optical input signal detected by an optical detective element in the course optical communications or the like is converted into a current input signal.
2. Description of Related Art
Recent years, infrared data communication (IrDA communication) function for connecting communication terminals with infrared rays has been applied to mobile terminals, personal computers, cellular phones, and the like. Furthermore, optical fiber communication networks have been well established as communication infrastructure. In such a communication system, an optical signal of infrared ray or the like is used as a digital signal. More specifically, an optical signal is converted into a current signal and the converted current signal is further converted into a voltage signal, thereby making it possible to detect presence/absence of the optical signal.
FIG. 13 shows a current-voltage converter 100 as a first related art. A pair of constant current source transistors M101, and M102 connect emitter terminals of transistors Q101 and Q102 and ground voltage GND. Base terminals of the transistor Q101, and Q102 connect to bias voltage source VBIAS. Diodes D101 and D102 connect collector terminals of the transistors Q101 and Q102 and power source voltage VCC. A photo diode PD for detecting an optical input signal connects to a connection point of the emitter terminal of the transistor Q101 and the constant current source transistor M101. Further on, connection points VM and VP connect the transistor Q101 and the diode D101, the transistor Q102 and the diode D102, respectively. These connection points VM and VP connect to a pair of differential input terminals of a differential amplifier circuit AMP101 to constitute a conversion voltage terminal VM and a reference voltage terminal VP for the current-voltage converter 100.
An optical input signal is detected by the photo diode PD, i.e., inputted as a current input signal Iin, and then, converted into a voltage value, and finally outputted to a next stage such as the differential amplifier circuit AMP101. It should be noted that the input terminal of the current-voltage converter 100 herein is shown as a single input. That is, as a circuit structure to conduct current-voltage conversion of an input signal Iin, the current-voltage converter 100 is provided with the diode D101, the transistor Q101, and the constant current source transistor M101. A similar circuit structure consisting of the diode D102, the transistor Q102, and the constant current source transistor M102 is also incorporated therein so as to determine an operational point of the current-voltage converter 100. Thereby, it is structured such that differential voltage between reference voltage VP corresponding to output voltage to be outputted to the reference voltage terminal VP and conversion voltage VM to be outputted to conversion voltage terminal VM is outputted as output voltage. Furthermore, as to the complimentary circuit, the connection point for the emitter terminal of the transistor Q102 and the constant current source transistor M102 can connect to a load such as a capacitance element. More specifically, the capacitance element is structured as a dummy terminal for making input loads of differential inputs same and corresponds to a the photo diode PD. Furthermore, a pair of current input signals complementary to each other can be inputted to obtain differential current signals.
Current converted into a current input signal Iin by the photo diode PD joins to a bias current IB1 flowing from the constant current source transistor M101 and flows to the diode D101 through the transistor Q101. An anode terminal of the diode D101 connects to the power source voltage terminal VCC. Therefore, forward voltage of the diode D101 having dropped is outputted to the conversion voltage terminal VM. The forward voltage of the diode D101 generates when the converted current flows there. On the other hand, a bias current IB2 flows coming from the constant current source transistor M102 flows into the diode D102 and dropping voltage equivalent to the forward voltage of the diode D102 obtained when the bias current IB2 from the power source voltage VCC flows thereto. Accordingly, the differential amplifier circuit AMP101 detects two inputs as differential voltage, namely, (1) the reference voltage VP outputted from the reference voltage terminal VP, and (2) the conversion voltage VM outputted from the conversion voltage terminal VM equivalent to voltage having dropped by the forward voltage which generates when a current input signal Iin flows to the diode D101 in comparison with the reference voltage VP.
In the current-voltage converter 100, the diode D101 converts a current input signal Iin into a form of logarithmic compression. Therefore, the conversion voltage VM for the conversion voltage terminal VM operates with an amplitude approximate to an operational point of about 0.7 V corresponding to forward voltage which makes diode conductive.
In general, a group of the diode D101, the transistor Q101, and the constant current source transistor M101, and a corresponding group of the diode D102, the transistor Q102, and the constant current source transistor M102 are structured with identical circuit elements, respectively. Theirs respective bias currents IB1 and IB2 are identical to each other.
FIG. 14 shows a current-voltage converter 200 as a second related art. In addition to the component elements of the current-voltage converter 100, the current-voltage converter 200 has a structure such that resistance elements R101 and R102 are connected in parallel to diodes D101 and D102 connected between collector terminals of transistors Q101 and Q102, and power source voltage VCC. Other than the above-mentioned partial structure, an essential circuit structure of the current-voltage converter 200 is similar to that of the current-voltage converter 100. Accordingly, in the second related art, same numerals are assigned to composing elements identical to the first related art and description of them will be omitted.
In the current-voltage converter 200, bias currents IB1 and IB2 flow to loads R101-D101 and R102-D102, respectively, wherein the resistance elements R101, R102 and the diodes D101, D102 are connected in parallel to one another. Accordingly, as to the load to which a current input signal Iin flows, current IB1+Iin mainly flows in the resistance element R1 until terminal-to-terminal voltage drop of a load reaches of about 0.7 V, forward voltage which makes the diode D101 conductive. A characteristic of conversion voltage outputted from a conversion voltage terminal VM varies in proportion to a current input signal Iin. After the current increases and the terminal-to-terminal voltage drop of a load reaches of about 0.7 V, the forward voltage of the diode D101, the current mainly flows in the diode D101 and the characteristic of the conversion voltage outputted from the conversion voltage terminal VM shifts to a characteristic of logarithmic compression against the current input signal Iin.
In general, a group of the diode D101, the transistor Q101, and the constant current source transistor M101, and a corresponding group of the diode D102, the transistor Q102, and the constant current source transistor M102 are structured with identical circuit elements, respectively. Theirs respective bias currents IB1 and IB2 are identical to each other. Further on, theirs respective resistance elements are also identical to each other.
However, in optical communications such as IrDA communication including infrared regions, an optical input signal dealt in the communications is a burst signal consisting of continuous pulse rows, in general. That is, a burst signal is a signal which is changeable in its pulse widths and duty ratio of pulses. Furthermore, intensity of light to be transmitted changes significantly depending on transmission distance or transmission environment of optical input signal. Accordingly, there has been a difficulty such that a current-voltage converter can hardly secure stable outputs of output voltage when converting current input signals into output voltage signals wherein optical input signals diverse in optical intensity are converted into current input signals.
Details will be described hereinafter. In the first related art, logarithmic compression processing is applied to whole input current range of current input signals, thereby to obtain output voltage.
Therefore, due to long distance between light source and a current-voltage converter, weakness of light intensity of light source, bad conditions for light propagation, or the like, there may be case such that micro current as a current input signal Iin is inputted and added to a bias current IB1 in the current-voltage converter 100. In such a case, voltage amplitude of converted output voltage results in a micro signal, whereby there arise problems such that noises are caused and normal output voltage signals are hard to be detected.
Furthermore, it is conceivable to lower bias current so as to detect output voltage signals accurately even when micro current is inputted. In such a case, a voltage value of an output voltage signal to which logarithmic compression is applied can be detected because a current input signal Iin to be added to a bias current IB1 can be made relatively larger. However, in such situation, a bias current IB1 is small compared with the current input signal Iin. Accordingly, bias state with respect to the diode D101, the transistor Q101, the constant current transistor M101 and the like in the current-voltage converter 100 changes significantly every switching of current input signal Iin. As a result, the current-voltage converter 100 cannot secure high-speed response capability. Further on, with respect to a current input signal Iin and conversion voltage VM for the conversion voltage terminal VM, waveform of a signal when being turned is likely to be deformed, and that of a signal when being terminated is likely to have tale phenomenon. As a result, response capability against state change deteriorates, which causes a problem such that response capability cannot keep up with high-speed frequency operation.
In the second related art, until terminal-to-terminal voltage of the resistance element R101 reaches forward voltage of the diode D101 (of about 0.7 V), sum current of a bias current IB1 and a current input signal Iin mainly flows in the resistance element R101. At this condition, a characteristic of conversion voltage of the conversion voltage terminal VM varies in proportion to a current input signal Iin. After the terminal-to-terminal voltage has reached the forward voltage of the diode D101 (of about 0.7 V), the sum current mainly flows in the diode D101 and the characteristic of the conversion voltage shifts to a characteristic of logarithmic compression. If bias currents IB1 and IB2 are set higher in this situation, the diode D101 shifts to a clamp state even if it is against a micro current input signal Iin. In the process of current-voltage conversion, voltage amplitude of an output voltage is converted into a very small signal through logarithmic compression processing. As a result, it becomes difficult to detect output voltage signals due to noises and the like. Therefore, to overcome the above problem, the bias currents IB1, IB2 can be set low. That is, with low bias current, a linear current-voltage conversion characteristic can be obtained for a micro current input signal Iin. However, with such current setting, it is not feasible to lower the differential voltage between the reference voltage terminal VP and the conversion voltage terminal VM to the forward voltage of the diode D101 (of about 0.7 V) when a large current input signal is inputted. Therefore, there arises a problem such that the input stage circuit structure of a next stage such as the differential amplifier circuit AMP101 is restricted.
The present invention is intended to solve the foregoing prior art deficiency. Its prime object is to provide a current-voltage converter capable of surely outputting an accurate voltage output signal regardless of current intensity when converting a current input signal with wide current range into a voltage output signal.
In order to achieve the above objective, a current-voltage converter that converts input current into output voltage that corresponds to differential voltage between conversion voltage outputted in response to input current and reference voltage, based on one aspect of this invention, comprises: a first current-voltage converting section that outputs the reference voltage derived from reference current; and a second current-voltage converting section that has a current-voltage conversion characteristic same as the first current-voltage converting section and outputs the conversion voltage in response to the input current, wherein both the current-voltage conversion characteristic of the first current-voltage converting section and that of the second current-voltage converting section are changed to compress conversion rate of the output voltage against the input current in case the input current is same as or higher than a predetermined current value.
In the current-voltage converter according to the one aspect of the present invention, the first current-voltage converting section and the second current-voltage converting section with the same current-voltage conversion characteristics output reference voltage and conversion voltage, respectively. In case input current is same as or higher than a predetermined current value, both the current-voltage conversion characteristic of the first current-voltage converting section and that of the second current-voltage converting section are changed to compress conversion rate of the output voltage against the input current and output the output voltage that corresponds to the differential voltage of the conversion voltage against the reference voltage.
Thereby, a current-voltage conversion characteristic appropriate for degree of input current can be set and optimal output voltage against wide input current range can be obtained. In case micro current is inputted as input current, change rate of current-voltage conversion characteristic is made large thereby making it possible to accurately detect input current without being influenced by noises therearound. Furthermore, conversion rate of the current-voltage conversion characteristic is made larger against input current same as or smaller than the predetermined current value where as it is made smaller against input current same as or larger than the predetermined current value. Thereby, output voltage range can be compressed to be narrow against wide input current region and an output voltage range suitable to a circuit structure of next stage can be set.
The above and further objects and novel features of the invention will more fully appear from following detailed description when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are purpose of illustration only and not intended as a definition of the limits of the invention.